'Darkness' is a utility spell in ADOM. It allows the caster to permanently darken the area around him/her. Darkenss is a ball spell and thus cannot be targeted. Some monster can't see in Darkness. In 1.1.1 they used to stand in one place even under attack unless panicking. This was fixed in pre7 and presumably these monsters just move randomly inside dark areas. Casting Darkness can be very dangerous to doomed characters due to possibility to be eaten by grue. For further details consult Darkness effect page.